Packaging materials wherein the gas barrier layer is made of aluminum or other metal foil, which resists the effects of temperature and humidity, have conventionally been used as barrier materials against oxygen, water vapor and the like, but the problem in this case is that it is not possible to see through the packaging materials and verify the contents. In recent years, therefore, attention has focused on transparent gas barrier films formed by depositing an inorganic oxide such as silicon oxide, aluminum oxide or the like by vacuum deposition, sputtering, ion plating, chemical vapor epitaxy or the like on a film substrate.
The problem is that such transparent gas barrier films are normally obtained by depositing an inorganic oxide on the surface of a substrate consisting of a tough, transparent biaxially oriented polyester film, left as is the deposited layer is vulnerable to abrasion and the like during use, and when the film is used as a packaging film, the gas barrier properties may be adversely affected by cracks in the inorganic that occur due to abrasion and stretching during subsequent printing or lamination or during filling with the contents, so satisfactory gas barrier properties cannot be achieved simply by inorganic oxide deposition.
Methods that have been proposed for solving this problem include a method laminating a polyvinyl alcohol with gas barrier properties over a metal oxide thin film (see for example, Patent Document 1), a laminated film obtained by coating the surface of a deposited layer of an inorganic compound with a coating agent consisting primarily of an aqueous solution or mixed water/alcohol solution of a water-soluble polymer and at least one of (a) 1 or more metal alkoxides and/or hydrolysates thereof and (b) tin chloride (Patent Document 2), a laminated film obtained by applying a coating composition consisting of a specific organosilane, a silyl group-containing fluorine polymer and an organopolysiloxane (Patent Document 3), a gas barrier coating film obtained by applying a coating agent comprising a polyvinyl alcohol-based resin and a metal alcoholate (Patent Document 4), and a method for manufacturing a gas barrier film by first forming a film of a composition consisting primarily of poly(meth)acrylic acid and a polyalcohol polymer, and then heat treating the film and immersing it in a medium containing a metal (Patent Document 5).
However, coated films consisting of alkoxides and the like are not very tough, and may crack or lose their gas barrier properties under conditions of use as packaging materials.
The oxygen barrier properties of barrier films obtained by laminating polyvinyl alcohol may decline under highly humid conditions, and in order for a composition of poly(meth)acrylic acid and a polyalcohol polymer to obtain barrier properties under highly humid conditions, it must undergo high-temperature treatment for at least 5 minutes at 180 to 200° C.    Patent Document 1: Japanese Patent Application Laid-open No. H6-316025 (claim 1)    Patent Document 2: Japanese Patent No. 2790054 (claim 1)    Patent Document 3: Japanese patent Application Laid-open No. 2000-63752 (claims 7, 11)    Patent Document 4: Japanese Patent Application Laid-open No. 2002-173631 (claims 1, 11)    Patent Document 5: Japanese Patent Application Laid-open No. H10-237180 (claim 15)